WATER DEFICIT AS A CLIMATIC DISCRIMINANT 13 



whatever the nature of the surface and subsoil (other than a sloping sheet of 

 impervious concrete or 'tarmac', which could dry between showers). Thus 

 no amount of artificial drainage could prevent waterlogging, and improv- 

 ing the land for arable agriculture is practically impossible. The natural 

 vegetation cover is blanket bog ('chmatic' bog) consisting of plants with 

 very shallow roots, growing on top of the decayed remains of their 

 predecessors. Lack of oxygenation of the waterlogged substratum prevents 

 any development of hving roots downwards. Only where slopes are too 

 steep for the dead or living vegetation to be retained (or where, because of 

 altitude, cold and wind enforce an upper hmit) is there no bog. Here there 

 is only bare rock, save for occasional very exceptional sites, where deeper 

 rooted plants may temporarily obtain a footing in rock crevices. In the 

 olden days, in the north-western Highlands and islands of Scotland, the 

 inhabitants hit on one of the few possible methods of very shghtly amehorat- 

 ing the conditions-by the laborious construction each year of 'lazy-beds', 

 deep ridges and furrows, which did permit of a certain amount of drainage, 

 and so of oxygenation, in the upper parts of the ridges. 



There does not at present exist a very satisfactory general map showing 

 the distribution of peat deposits in Scotland, but Fig. i shows the distribu- 

 tion of 'improved land' (i.e. arable and ley), which is complementary to it. 

 It wiU be seen that -excluding hill and mountain-it corresponds with the 

 map (Fig. 2) showing the observed PWD in 1957- a year probably fairly 

 close to the average. On the basis of the known distribution of 'chmatic' 

 peat, and of 'improved land', one can tentatively insert the average line of 

 no significant PWD (less than ^-i in. or 13-25 mm calculated on a calendar 

 month basis, in an average year) as shown on this map. Almost the only 

 satisfactory arable land in the north-west is on the coastal 'machair'- 

 calcareous dune sand -forming a relatively fertile porous soil, where it can 

 be saved from wind-blow. But it should be noted that, with evaporation 

 augmented by strong winds, and shghtly less summer rainfall than occurs a 

 httle way inland, there are probably 'outhers' of rather greater PWD here 

 also. 



Many places with small PWD will be in the same state as those with no 

 PWD, because there is no practicability of artificial drainage. Almost all of 

 north-western Scotland is in tliis position, the few areas of cropland being 

 precisely at those places where the local physiography and geology specially 

 favour drainage. Places with a larger PWD in an average year would, it is 

 suggested, develop bog only where drainage is impeded, and if this 

 impedance is removed, arable cultivation becomes possible. This is the case 

 in most lowland areas in the east and south of Scotland, where it is on 



